This technique is implemented in a vessel filled with an acoustic coupling liquid such as water, using an ultrasound transducer that is immersed in the water and that is pointed towards the part to be inspected perpendicularly to a surface thereof, the transducer being separated from the surface of the part by a depth of water referred to as the “water column”.
The transducer emits a focused beam of ultrasound waves, some of which are reflected at the surface of the part, while the remainder are transmitted to the inside of the part, the transmitted ultrasound waves possibly encountering an internal defect capable of reflecting them on their path within the part. The echoes coming from these reflections are picked up by the transducer, and the amplitudes of the echoes represent the magnitudes of the detected defects.
In practice, ultrasound inspection of a part involves relative displacement between the part and the transducer or a series of transducers serving to scan the part by one or more focused ultrasound beams.
When a focused ultrasound beam comes progressively closer to the edge of the part, an ever-increasing fraction of the beam passes beyond the edge of the part such that a fraction of the energy of the focused beam is emitted outside the part and cannot be reflected by a defect in that portion of the part which is covered by the focused beam.
Consequently, a defect that is detected in the form of an echo will be detected with amplitude that becomes ever-smaller on the focused beam getting closer to the edge of the part, with said amplitude becoming zero when the focused beam no longer encounters the part.
The fraction of the part in which this phenomenon leads to a decrease in the amplitude of the echo produced by reflection of the focused beam on a defect, constitutes a zone that cannot be inspected by known methods, and it is referred to as a “shadow” zone or as a “lateral dead” zone of the part.
The prior art technique does not enable this lateral dead zone to be eliminated over the full depth of the part, and, by precaution, it is necessary to give it some over-dimensioned potential value, e.g. 12 millimeters (mm), even though it is in fact smaller than that, e.g. half as wide, and that corresponds to a relatively large loss of material.